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Scientists have developed a strap-on gadget that generates electricity as you bend your knees and walk.

The knee brace generates enough power to charge up 10 mobile phones at once, the US and Canadian scientists report today in the journal Science.

Researchers have been working on ways to harness the motion of the human body to create power.

A shoe-mounted device was nice and light, but did not generate much electricity.

A backpack device that generated power as it bounced up and down while a person walks generated a lot of electricity, but was heavy to lug.

The new energy-capturing knee brace, its inventors say, seems to find a happy medium, generating decent amounts of power while still being relatively light.

The scientists envisioned numerous applications for such a device. It could be of value to hikers or soldiers who may not have access to electricity, they say.

It also could be built into prosthetic knees or other implantable devices whose users must occasionally have surgery to replacement the batteries.

Associate Professor Arthur Kuo, a University of Michigan mechanical engineer who worked on the device, says it functions a bit like how regenerative braking charges a battery in hybrid cars.

These regenerative brakes collect kinetic energy that normally dissipates as heat when the car slows down.

The knee device collects energy lost when a person brakes the knee after swinging the leg forward to take a step, the researchers say.

"It generates a fairly substantial amount of power compared to previous devices and it does so in a way that doesn't affect the user very much," Kuo says.

"You could easily power 10 [mobile] phones at once. There are some low power computers that you could power. You could imagine devices like GPS locaters, satellite phones," he says.

With a device placed on each leg, volunteers walking on treadmills generated about 5 watts of electricity walking at a leisurely 3.5 kilometres per hour.

Each of the devices weighs about 1.6 kilograms, which Kuo says is still too unwieldy.

"Even though we've demonstrated this new way to generate power, we don't mean to say this is a useable product at this time. The principle limitations are that our prototype is pretty heavy and bulky," Kuo says.